Fast shutter for protection from electromagnetic radiation
BACKGROUND OF THE INVENTION 
     This invention relates to modulation of electromagnetic energy and more particularly to a device for modulating incidient electromagnetic energy known as electromagnetic pulse (EMP). In still gretare particularity the invention relates to a shutter device for selectively blocking the passage of EMP upon detection thereof while allowing electromagnetic waves having frequencies gretare than the EMP to pass through the shutter. 
     Protection against that range of electromagnetic waves known as microwave and radiowave is highly desirable. Even more desirable is protection against that range of energy known as EMP. EMP is electromagnetic wave energy associated with nuclear detonation and is normally of large amplitude falling in a frequency range of 0.1 MHz to 10 MHz. If left unprotected, electronic equipment can be adversely affected if exposed to EMP. Consequently, there is a need for the shielding of such equipment. However, because some electronic equipment is used to detect or operate in response to electromagnetic waves of selective frequencies or ranges other than EMP, any shielding device must preferably be transparent to these harmful frequencies of interest. Heretofore, shields that block EMP have not had the desired transparency over broad unharmful frequencies ranges of interest. 
     The shutter described herein takes advantage of the properties common to those materials known as chalcogenide alloy glasses which are made from the Group V or VI elements. The general system consists of Te-As-Ge-Si. While not to be considered limited thereto, one specific alloy of this general system which lends itself to the structure disclosed hereinbelow is Te 40  As 35  Ge 7  Si 18 . These alloys are known as amorphous semiconductors exhibiting which is commonly called &#34;memory switch,&#34; meaning that when in an amorphous state, they exhibit high resistance and act as a resistor, and when in a microcrystalline state, they exhibit low resistance and act as a metal. The literature has reported this property and their ability to switch rapidly from a resistor to a metal. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is a primary objective of the invention to provide for a device that blocks EMP. 
     Another primary objective of the invention is to provide for a device that protects electronic equipment and personnel from harmful EMP. 
     Another object of the invention is to provide for a device that blocks EMP and allows passage of other electromagnetic wave frequencies. 
     A further object of the invention is to provide for a device that is normally transparent to all electromagnetic wave frequencies and switches to selectively prevent passage of a sensed EMP. 
     And, a still further object of the invention is to provide for a device that reverts to a transparent state upon the termination of the EMP. 
     The objects are achieved by providing for a high speed solid state electronic switch that functions as a shutter for blocking the passage of EMP in response to detection thereto by an antenna. The shutter includes thin films of amorphous semiconductor chalcogenide alloy glass from the Group V or VI elements deposited on opposite surfaces of a silicon dioxide insulator. Electrodes are arranged on the insulator surfaces in electrical contact with the thin film surfaces and define an aperture for passing incident electromagnetic waves through the amorphous semiconductor material. An antenna tuned to detect incoming EMP is coupled to a trigger circuit which amplifies the antenna signal and discharges a capacitor across the semiconductor material. The applied voltage causes the material to switch from a state of high resistance to a state of low resistance. The material exhibits properties of a metal and opacity to the incident EMP while remaining substantially transparent to frequencies greater than that of the EMP. Removal of the applied voltage within a predetermined time frame causes the material to revert to its state of high resistance and transparency to all frequencies. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is side elevation of the preferred embodiment showing details of construction. 
     FIG. 2 is a front elevation of the preferred embodiment showing details of construction. 
     FIG. 3 is a schematic of an antenna and trigger circuit for use with the preferred embodiment of FIGS. 1 and 2. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Shown in FIGS. 1 and 2 is a shutter 10 including a substrate 12 of preferably silicon dioxide onto which thin film alloys 24, 26 of amorphous semiconductor material are deposited by normal sputtering techniques. As heretofore set out the thin film amorphous semiconductor material is a chalcogenide alloy glass chosen from the general system consisting of the Group V or VI elements Te, As, Ge, Si. For the preferred embodiment disclosed herein, it is contemplated that the alloy Te 40  As 35  Ge 7  Si 18  is acceptable. Also substrate 12 is not to be considered limited to silicon dioxide but rather any insulative material that is transparent to the unharmful electromagnetic waves of interest may be used. Also deposited onto the opposing surfaces of substrate 12 are four electrodes 14, 16, 18, 20. First electrode pair 14, 16 are deposited on one surface of substrate 12 so as to be in electrical contact with first thin film 24. Likewise, second electrode pair 18, 20 is deposited on the opposite surface of substrate 12 so as to be in electrical contact with second thin film 26. In the embodiments shown, thin films 24, 26 are shown deposited overlapping the respective electrodes to create the required electrical continuity. Electrodes 14, 16, 18, 20 can be made of carbon or a metal that will not alloy with thin films 24, 26. An acceptable electrode material is platinum. Electrodes 14, 16, 18, 20, define a square or rectangular aperture 22, see FIG. 2, for passing incident electromagnetic waves through the thin films. The aperture width as measured between the electrode pairs should be kept to a minimum, and in any case should not be less than the wavelength of the normal signal of interest. Typically, the aperture width would be from 1 to 100 microns. More specifically, assuming the signal of interest to be allowed to pass through the shutter is in the visible range, the gap should be not less than the wavelength of the visible signal. For aperture widths falling in the 1 to 100 micron range, signals of interest are restricted to those having wavelengths less than 10 -4  meters. The thickness of thin films 24, 26 typically fall in the range of 1 to 100 microns and is dependent on the expected intensity of the EMP to be encountered. 
     The film thickness is adjusted typically within the 1 to 100 micron range to assure prevention of passage of the EMP with thicker films offering greater blocking capabilities. 
     As shown in FIG. 1, there is an electromagnetic wave sensor in the form of an antenna 36 tuned to detect and generate a signal in response to EMP. Antenna 36 is located in close proximity to the line of sight through aperture 22 so as to detect the EMP incident on the shutter. Antenna 36 is coupled to a shutter trigger circuit 40 via a cable 38. Likewise, the outputs of trigger circuit 40 are coupled to electrode pairs 14, 16 and 18, 20 via cables 28, 30 and 32, 34 respectfully. It is to be understood that the trigger circuit outputs are connected to the respective electrode pairs such that the current flow through first electrode pair 14, 16, and thin film 24 is in opposite direction to the current flow through second electrode pair 18, 20 and thin film 26. The oppositely directed current flows are necessary so that the magnetic fields developed cancel each other. Also, all cables such as leads 28, 30, 32, 34 and antenna cable 38 should be kept as short and symmetric as possible. 
     Trigger circuit 40 is designed to be inoperative at the amplitude levels of the signals of interest to allow their unimpeded passage through the shutter. Trigger circuit 40 becomes operative when the foot of an EMP is detected. The foot of an EMP is that portion of the EMP wave defined by the time interval required for the amplitude of the wave to rise to a predetermined intersity level. When the amplitude of the EMP reaches the operational level of the trigger circuit, the EMP signal is amplified by amplifier 42 and a capacitor 44 is caused to discharge across the respective electrode pairs, thereby applying a voltage across the thin film semiconductor materials. 
     As set out above, the amorphous semiconductor materials used in this invention normally exhibit high resistance. However, when sufficient voltage, due to the discharge of capacitor 44, is applied across the material, it exhibits a low resistance and assumes properties of a metal effectively blocking passage of the EMP. All of the amorphous semiconductor materials for use herein have a threshold voltage at which they switch to the low resistance state. Normally a threshold voltage on the order of 10 5  v/cm is enough to cause the material to switch to the low resistance, EMP blocking, state. Typically, there is a time delay of approximately 10 -6  sec., between the application of the threshold voltage across the film material and the instant the material begins to switch to the state of low resistance. After the initial time delay, it typically takes only 10 -10  sec. for the material to complete the transition to the low resistance state and become opaque to the EMP. 
     It can be seen that the delay period of 10 -6  sec , is relatively long, and, as set out below, may be sufficiently long, to allow appreciable EMP to pass through the shutter before it can switch to the blocking state. This delay period can be significantly reduced by increasing the applied voltage across the material to a level above the threshold voltage of the material. For example, increasing the applied voltage to about 100% of the threshold voltage reduces the delay period to about 10 -9  sec. The time delay period decreases exponentially after the applied voltage exceeds the threshold voltage by about 20%. Increasing the applied voltage does not effect the time period required for the material to completely switch to the low resistance state once it begins the transition. As stated, this time period is relatively fast and on the order of 10 -10  sec. 
     Additionally, the shutter will automatically revert back to the high resistance state if the applied voltage is removed within approximately 10 -3  sec. after the material begins to switch. If the applied voltage is not removed within this 10 -3  sec. period, the film becomes set in the low resistance state even if the applied voltage is later removed. Therefore, if the shutter is required to remain closed for a period longer than 10 -3  sec., for example, when an EMP of long duration is sensed, a reset pulse of about 120 mA current of 5 microseconds duration must be applied to the films to cause them to revert back to their high resistance transparent state. Circuits for accomplishing this are readily designed by those skilled in the art having the benefit of the above disclosed requirements. 
     As stated, the trigger circuit must detect the foot of the EMP in order to trigger the switching of the films to the low resistance state. If the wavelength of incident EMP is sufficiently short, the foot of the EMP wave may be less than the switching time of the shutter, thereby allowing the higher intensity level portions of the EMP wave to pass through the shutter before the films can switch to the opaque state. If the switching time of the shutter is too slow, and/or the foot of the EMP wave too short, the intensity level of the EMP wave thereby allowed to pass through the shutter may exceed acceptable limits. 
     Therefore, the shutter must be constructed to switch as fast as possible. Factors such as minimum cable lengths, minimum circuit path lengths, fast operating circuit components, and applied voltages above the threshold level of the amorphous semiconductor thin films are required to minimize the switching delay period. 
     Further, it is thought that the foot of the EMP will be of an acceptable magnitude to allow sufficient time for the trigger circuit to switch the shutter without allowing the higher intensity portions of the wave to pass therethrough when the EMP has a wavelength greater than 200M. For wavelengths less than 200M the rate of the rise in amplitude of the EMP wave is sufficiently fast so as to allow the intensity of the EMP to rise to unacceptable levels prior to the circuitry and film responding to switch to the opaque state. 
     While the embodiment described operates for EMP wavelengths greater than 200M, it is contemplated that those skilled in the art having the benefit of the teachings herein can readily modify and adapt the invention to switch sufficiently fast to prevent the passage of EMP of shorter wavelengths, therefore this invention is not to be considered to be limited to wavelengths greater than 200M and said modifications are to be within the scope of the appended claims.